Task 1

Introduction

The snowshoe hares (Lepus americanus), named after its big hind feet, is a keystone prey species in northern boreal forests in North America. They are shy and secretive and prefer habitats with a dense shrub layer. The rabbits active year-round and turn their fur white during the winter and rusty brown during the summer for camouflage.

Snowshoe hares in winter

Snowshoe hares in winter

Snowshoe hares in summer

Snowshoe hares in summer

Picture in winter was downloaded from Cottage Life and the one in summer was from Montana Public Radio.

Data Source and Method

I used capture-recapture data of snowshoe hares at five locations in the Bonanza Creek Experimental Forest of Tanana Valley, Alaska to explore how the weight of snowshoe hares is related to sex and changed with time. The data was provided by Bonanza Creek LTER and its time scale ranged from 1999 to 2012, where Snowshoe hares densities peaked in 1999 then declined (Kielland et al. 2017). I looked at the distribution and summary table of weight from 1999 to 2012 and track the weight of 11 individuals captured-recaptured more than 15 times.

Made from Google Earth Engine on Feb. 4, 2020.

Result

The distributions of snowshoe hares weight from 1999 to 2012 were plotted for both males and females (Figure 1).

Figure 1: Distributions of snowshoe hares weight for males and females (1999-2012). The weight of snowshoe hares were normally distributed and peaked around 1500g for both sex. There were slightly more females (red) than males (green). Data source: Bonanza Creek LTER

I summarized the means, medians, standard deviations, and number of observations of weight of snowshoe hares for females and males (Table 1). On average, males weighted sligthly less than females.

Table 1: Summary of snowshoe hares weight for males and females (1999-2012). The weight of female snowshoe hares (mean = 1367.43, sd = , n = 1716) did not differ much from males (mean = 1339.57, sd = , n = 1207). Data source: Bonanza Creek LTER

Sex Mean (g) Median (g) Standard Deviation (g) Sample Size
Female 1367.43 1420 345.68 1716
Male 1339.57 1380 305.13 1207

Track individuals

There were 11 individuals with more than 15 observations that were tracked. The corresponding time range spanned from 2003 to 2011. Mean weights were calculated for observations in months and years when the 11 individuals were captured/recaptured. The trend of changes in mean weight goes relatively consistent in either gender groups, especially for Male #163 and #92, Female #246 and #233, and Female #969, #737 and #729 (except year 2011) (Figure 2). Most of the 11 individuals weighted more than the population means at various time spots of sampling. In general, most of the 11 individuals had their weights increased when they were recaptured. The results could have represented a survivor bias where individuals couldn’t live well had a lower chance of being recaptured.

Figure 2: Track 11 individual’s weight of snowshoe hares for both males and females (2003-2011). The graph included 8 females and 3 males that had more than 15 capture-recapture records and labeled their unqiue animal identifiers in boxes. The population means (2003-2011) for males and females were plotted as dotted line and had the value labeled. Most indivduals in this subset weighted more than the population averages. Data source: Bonanza Creek LTER

Reference:

Kielland K., F. S. Chapin, R. W. Ruess. 2017. Snowshoe hare physical data in Bonanza Creek Experimental Forest: 1999-Present. Environmental Data Initiative. https://doi.org/10.6073/pasta/03dce4856d79b91557d8e6ce2cbcdc14.